Recently, Evolved Universal Terrestrial Radio Access (hereinafter, referred to as “EUTRA”) aimed at high speed communication has been examined by a standardized group, 3rd Generation Partnership Project (3GPP), by introducing technology examined for a 4th generation frequency bandwidth into a 3rd generation frequency bandwidth (Non-Patent Document 1).
In EUTRA, as a communication method, Orthogonal Frequency Division Multiplexing Access (OFDMA) is used. OFDMA is a communication method which is strong against multi-path interference and suitable for high speed transmission. In addition, a specification for an operation of an upper layer in EUTRA includes low latency and low overhead and simplified technology is being employed. The operation of the upper layer includes data transmission control, resource management control, and so on.
In a cellular mobile communication method, a mobile station device needs to be wirelessly synchronized with a base station device in advance in a cell or a sector. For this reason, the base station device transmits a synchronization channel (SCH) having a defined structure to the mobile station device. Then, the mobile station device is synchronized with the base station device by detecting the synchronization channel (SCH). The cell or the sector is a communication area of the base station device.
In EUTRA, as the synchronization channel (SCH), a P-SCH (primary SCH, primary synchronization channel) and an S-SCH (secondary SCH, secondary synchronization channel) are prepared. Each cell (or sector) is identified by the mobile station device by the use of a cell ID determined by the signals of the primary synchronization channel (P-SCH) and the secondary synchronization channel (S-SCH).
The cell ID is determined by a combination of 3 types of primary synchronization channels (P-SCHs) and 168 types of secondary synchronization channels (S-SCHs), for a total of 504 (3×168=504).
FIG. 16 is a flowchart showing a process in a conventional cell search method.
The mobile station device performs a P-SCH identification process by correlating a replica signal of a primary synchronization channel (P-SCH) with a received signal. Accordingly, the mobile station device acquires slot timing (step S1).
The mobile station device then performs an S-SCH identification process by correlating a replica signal of a secondary synchronization channel (S-SCH) with the received signal. Accordingly, the mobile station device acquires frame timing by an acquired transmission pattern of the secondary synchronization channel (S-SCH). In addition, the mobile station device acquires a cell identification (ID) for identifying the base station device (step S2).
Such a series of control, that is, a step control up to the mobile station device executing wireless synchronization with the base station device and specifying the cell ID of the base station device is referred to as a cell search procedure.
In a cellular mobile communication method including EUTRA, a mobile station device communicates with a base station device within a cell (or a sector) that is a communication area of the base station device. When the mobile station device is wirelessly connected to a certain base station device, a cell where the mobile station device is located is referred to as a serving cell. Meanwhile, a cell located around the serving cell is referred to as a neighboring cell.
The mobile station device can determine a cell having a satisfactory quality by measuring and comparing the reception qualities of the serving cell and the neighboring cell. A process in which the mobile station device moves from the serving cell to the neighboring cell to change a cell to which the mobile station device is wirelessly connected is referred to as a handover.
In this case, a signal used for the mobile station device to determine the levels of reception qualities of cells is referred to as a downlink reference signal. The downlink reference signal is a predetermined signal sequence corresponding to a cell ID. That is, it is possible to uniquely specify a downlink reference signal simultaneously transmitted from a cell by identifying a cell ID of the cell (Non-Patent Document 1).
FIG. 17 is a diagram showing an example of a configuration of a radio frame in EUTRA. In FIG. 17, the horizontal axis indicates time, and the vertical axis indicates frequency. The radio frame consists of an area (a shaded area of FIG. 17) including a predetermined frequency area (BR) and a predetermined transmission time interval (slot) as one unit (Non-Patent Document 1). The frequency area (BR) is the assembly of a plurality of sub-carriers arranged on the frequency axis.
The transmission time interval consisting of the integer times of one slot is referred to as a sub-frame. A combination of a plurality of sub-frames is referred to as a frame. In FIG. 17, one sub-frame consists of two slots.
An area (the shaded area of FIG. 17) divided by the predetermined frequency area (BR) and the length of one slot is referred to as a resource block. In addition, one frame consists of 10 sub-frames. BW of FIG. 17 indicates a bandwidth of a system, and BR indicates a bandwidth of the resource block.
FIG. 18 is a flowchart showing a handover procedure used in EUTRA. FIG. 18 shows a control operation in which the mobile station device communicates with a cell of a handover source (hereinafter, referred to as a source cell), and a handover to a cell of a handover destination (hereinafter, referred to as a neighboring cell) is performed.
The following procedure will be described on the assumption that the cell ID of the source cell is CID_A, and the cell ID of the neighboring cell is CID_B. Here, the mobile station device receives each of the downlink reference signals of the CID_A and the CID_B from each of the base station device having the cell ID of CID_A and the base station device having the cell ID of CID_B (steps S001 and S002). Then, the mobile station device measures the reception quality acquired from each of the downlink reference signals.
The mobile station device then performs a measurement report process (step S003). That is, the base station device having the cell ID of CID_A is notified of the measurement result of the mobile station device as a measurement report message (step S004). The base station device having the cell ID of CID_A determines whether the handover to the base station device having the cell ID of CID_B is necessary based on the contents of the measurement report message. When it is determined that the handover is necessary, the base station device having the cell ID of CID_A notifies the mobile station device of the necessity of the handover to the base station device having the cell ID of CID_B using a handover request message (step S005), and requests the preparation for the handover.
When it is determined that the handover can be performed, the base station device having the cell ID of CID_B that has received the handover request message notifies the base station device having the cell ID of CID_A of a handover request permission message (step S006).
The base station device having the cell ID of CID_A that has received the handover request permission message notifies the mobile station device of a handover command message (referred to as a handover command) (step S007).
When the mobile station device receives the message, the handover process starts (step S008). When a handover execution time is included in the handover command message, the mobile station device performs the handover when the handover execution time lapses.
In some cases, immediate execution may be designated as the handover execution time. The mobile station device changes a control parameter of a transmission/reception circuit or a radio frequency designated by the handover command message at the handover execution time. Subsequently, the mobile station device performs a downlink synchronization setup process for setting up downlink wireless synchronization with the base station device having the cell ID of CID_B (handover process).
The control parameter for the downlink synchronization setup process is included in the prior handover command message, or the mobile station device is informed or notified thereof in advance by the cell having the cell ID of CID_A. After the downlink synchronization setup is completed, the mobile station device performs a random access transmission so as to set up uplink synchronization with the cell having the cell ID of CID_B (step S009). This process may be called a handover access.
In fact, the random access is performed by using a (contention-based) channel which may cause conflict. However, a method is proposed which allocates a preamble sequence (dedicated preamble) in the handover command message to each mobile station device in advance, for the purpose of the (contention-free) random access transmission not causing conflict (Non-Patent Document 2).
The mobile station device performs the random access transmission using the preamble sequence designated in the handover command message. The base station device having the cell ID of CID_B that has received the preamble sequence determines that the handover of the corresponding mobile station device is completed. Then, the base station device having the cell ID of CID_B notifies the mobile station device of uplink resource allocation information for transmitting a handover completion message (which may be called a handover confirm) and uplink synchronization information for adjusting the uplink transmission timing (step S010).
The mobile station device adjusts the uplink transmission timing based on the above-described information, transmits the handover completion message to the base station device having the cell ID of CID_B using the designated uplink resource, and then completes the handover (step S011).
In addition, whether the downlink reference signal is described as downlink reference signal (Reference signal) or DL-RS (Downlink Reference signal) in Non-Patent Document 1, the meaning is the same.
However, in the cell search procedure and the handover procedure of the conventional art, when the same cell ID is allocated to the base station device, that is, a plurality of base station devices having the same primary synchronization channel (P-SCH) and secondary synchronization channel (S-SCH) are in an area, it is not possible to guarantee the operation of the mobile station device in that area. This will be described with reference to FIG. 19.
FIG. 19 is a flowchart showing the handover procedure when the same cell ID is measured by the mobile station device. FIG. 19 shows a case where the conflicting cell having the same cell ID (CID_B) as the neighboring cell of FIG. 18 is in the same area.
Here, the mobile station device receives the downlink reference signal from each of the source cell (CID_A), the neighboring cell (CID_B), and the conflicting cell (CID_B) (steps S020, S021, and S022). Then, the mobile station device measures the reception quality acquired from each of the downlink reference signals.
However, in the cell search procedure of the conventional art, since the mobile station device can detect only 2 types of cell IDs of CID_A and CID_B, the mobile station device cannot recognize if there are two cells having the same cell ID (CID_B).
For this reason, the downlink reference signals of the neighboring cell (CID_B) and the conflicting cell (CID_B) are measured without distinguishing them, and the CID_A is notified of the measurement report message (step S024) according to the measurement report process (step S023).
That is, when the neighboring cell (CID_B) and the conflicting cell (CID_B) are synchronized, the synthesized downlink reference signal is measured in the mobile station device. In addition, when the neighboring cell (CID_B) and the conflicting cell (CID_B) are not synchronized, one of the downlink reference signals is determined as a delay wave in the mobile station device.
In the state where the cell IDs conflict with each other as shown in FIG. 19, a plurality of problems arise. For example, the source cell (CID_A) cannot recognize whether the reception quality of the CID_B cell included in the measurement report message from the mobile station device belongs to the neighboring cell (CID_B) or the conflicting cell (CID_B).
In addition, when it is reported that the reception quality of the CID_B cell is acquired by synthesizing the reception qualities of the neighboring cell (CID_B) and the conflicting cell (CID_B), the reception quality cannot be suitably used for the handover reference. Further, even when the source cell (CID_A) transmits the handover command message to the neighboring cell (CID_B) to the mobile station device, it is not possible to clearly designate the neighboring cell (CID_B) as the cell of the handover destination. Accordingly, there is a possibility of the mobile station device executing the handover to the conflicting cell (CID_B).
In Non-Patent Document 3, in order to solve the problem caused by the same cell IDs, a method is disclosed in which unique IDs (Global Cell Identity, hereinafter, referred to as “GCID”) prepared sufficiently more than at least 504 types of cell IDs are allocated to all cells together with the cell IDs.
Because of using the GCID, it is possible to identify conflicting cells that cannot be identified by only the cell ID, using the GCID. In addition, by designating the cell of the handover destination using the GCID in the handover command message, it is possible to clearly designate the cell of the handover destination of the mobile station device.
In the method using the GCID shown in Non-Patent Document 3, even when the same cell ID is allocated to at least two cells (conflicting cells) in the measurement area, the mobile station device needs to autonomously identify the conflicting cell based on the synchronization channel (SCH) and the downlink reference signal.
However, it is difficult for the mobile station device to autonomously detect the allocation of the same cell ID, and divide and receive each of the radio signals.
For this reason, when there is a conflicting cell, a problem arises in that the process in the communication between the mobile station device and the base station device becomes complicated.    Non-Patent Document 1: 3GPP TS (Technical Specification) 36.211, Physical Channels and Modulation. V8.0.0 (http://www.3gpp.org/ftp/Specs/html-info/36211.htm)    Non-Patent Document 2: 3GPP TS 36.300, Overall description; Stage 2.V8.3.0 (http://www.3gpp.org/ftp/Specs/html-info/36300.htm)    Non-Patent Document 3: Huawei, “Detection of conflicting cell identities”, R3-071947, 3GPP TSG-RAN WG3 Meeting #57 bis, Sophia Antipolis, France, 8-11 Oct. 2007